1. Field of the Invention
This invention relates to the control of oil reservoir permeability utilizing a high temperature profile gel having a controllable delay mechanism.
2. Background Information Including Description of Related Art
When hydrocarbon producing wells are drilled, initial hydrocarbon production is usually attained by natural drive mechanisms, e.g., water drive, solution gas, or gas cap, which force the hydrocarbons into the producing wellbores. If a hydrocarbon reservoir lacks sufficient pore pressure as imparted by natural drive to allow natural pressure-driven production, artificial lift methods, e.g., pump or gas lift are used to produce the hydrocarbon.
As a large part of the reservoir energy may be spent during the initial or "primary" production, it is frequently necessary to use secondary hydrocarbon production methods to produce the large quantities of hydrocarbons remaining in the reservoir. Waterflooding is a widespread technique for recovering additional hydrocarbon and usually involves an entire oil or gas field. Water is injected through certain injection wells which are selected based on a desired flood pattern and on lithology and geological disposition of the pay interval. Displaced oil is then forced into producing wells in the field.
Advancements in secondary hydrocarbon producing technology has led to several improvements in water-flood techniques. For example, the viscosity of the injected water can be increased using certain polymer vicosifiers, such as polyacrylamides, polysaccharides and biopolymers, to improve the "sweep efficiency" of the injected fluid. This results in greater displacement of hydrocarbons from the reservoir.
Ability to displace oil form all the producing intervals in a hydrocarbon reservoir is limited by the lithological stratification in the reservoir. That is, there are variations in permeability which allow the higher permeability zones to be swept with injected fluid first and leave a major part of the hydrocarbon saturation in the lower permeability levels in place. Thus, one of the problems encountered in the water flooding of permeability-stratified reservoirs is the formation of water channels through the most permeable routes between the injector and producer wells that greatly reduces the sweep efficiency of the driving fluid. This situation is also common in other fluid and gas-driven oil recovery processes, such as CO.sub.2 and steam flood. In dealing with the problem, the permeability of such channels must be reduced to improve the sweep efficiency. This approach is commonly called permeability profile control. One of the methods practiced in the field is to inject gelatin solutions into these high permeability channels to form a solid gel plug after the solution is set in these channels.
Gel placement and its stability are two important considerations for a successful profile control process, especially in the treatment of high temperature reservoirs due to the harsh environment and fast gelation rate. However, there is need for more hydrothermally stable polymers for a long, reliable service and the employment of better gelation chemistry so that gel will not set prematurely during injection.
U.S. Pat. No. 4,896,723, issued Jan. 30, 1990 to Hoskin et al., discloses a cross-linked polymer obtained by cross-linking polyvinyl alcohol with a mixture of a phenolic component and an aldehyde or a mixture of a naptholic component and an aldehyde, utilized as a permeability control agent in oil recovery operations.
U.S. Pat. No. 5,015,400, issued May 14, 1991 to Shu, teaches the use of amino resins such as melamine-formaldehyde resin to co-gel and crosslink with polymers having amine, amide, hydroxyl or thiol functionalities, e.g., polyvinylalcohol, or acrylamide-modified polyvinylalcohol, in the formation of gels useful as permeability control agents for high temperature reservoirs during a water-flooding operation in an oil recovery process.
U.S. Pat. No. 5,134,176, issued Jul. 28, 1992 to Shu, discloses gel-forming compositions capable of plugging highly permeable zones in a subterranean formation and comprising water, a viscosifying amount of a water-dispersible polyvinylamine copolymer, and a cross-linking agent which is a mixture of an aldehyde and a phenolic component in an amount effective to cause gelation of the aqueous solution of the copolymer.